Language extensions

Language extensions are used to enable language features in Haskell that may seem useful in certain cases. They can be used to loosen restrictions in the type system or add completely new language constructs to Haskell.

They can be enabled using the LANGUAGE pragma

{-# LANGUAGE <Extension>, <Extension> #-}

or (in GHC) using flags -X<Extension>.

Before just using the language extension that fits your need, think about when it is useful and what risk it may bring to your program.

List of language extensions by name

This list is far from complete and needs extension.

• DatatypeContexts : Add type constraints to your datatype.
• DefaultSignatures
• Derive:
  • DeriveDataTypeable
  • DeriveGeneric
  • DeriveFunctor
  • DeriveFoldable
  • DeriveTraversable
  • GeneralizedNewtypeDeriving
• DuplicateRecordFields (GHC 8.0.1+) : Allow definition of record types with identically-named fields.
• ExistentialQuantification
• FlexibleContexts
• FlexibleInstances : Allow a type parameter to occure twice within a type class instance.
• FunctionalDependencies : Are used to constrain the parameters of type classes.
• GADTs : Generalised algebraic datatypes - A more general approach to algebraic datatypes.
• ImplicitParams
• KindSignatures
• MultiParamTypeClasses : Enable multiple type parameters in type classes.
• NoMonomorphismRestriction
• OverlappingInstances
• Rank2Types
• RankNTypes
• ScopedTypeVariables
• TemplateHaskell
• The GHC Users Guide has section on TupleSections. It allows tuples to be partially applied.
• TypeFamilies
• UndecidableInstances
• ViewPatterns
• QualifiedDo

See also

• Use of language extensions